Generally, a vertical-type mill has a drive portion which drives a milling table to rotate, a milling portion which mills a raw material (for example, coal) and obtains solid particles by meshing of the milling table with a milling roller or a milling ball, a classification portion which is placed above the milling portion and which has a classifier for classifying the milled solid particles and a recovery cone (hopper) for collecting coarse particles classified and dropped downward and guiding the coarse particles to the milling portion, a distribution portion which distributes fine particles sent from the classification portion to a supply destination (for example, a boiler), and a coal feeding pipe (raw material feeding pipe) through which the raw material can be inputted from above the milling table (see Patent Literature 1).
The raw material supplied from the coal feeding pipe falls down to the central portion of the milling table. Since the milling table is rotating, the raw material falling down onto the milling table moves on the milling table toward the outer peripheral portion thereof while drawing a spiral locus due to centrifugal force caused by the rotation. Then, the raw material is bitten and milled between the milling table and the milling roller or the milling ball. Milled solid particles are blown upward while dried by hot air introduced from a throat provided around the milling table. The solid particles blown upward are classified by the classifier. Coarse particles larger than a predetermined particle size fall down into the recovery cone and are inputted onto the milling table again. On the other hand, fine particles not larger than the predetermined particle size are passed through the classifier and then conveyed to a predetermined supply destination by the distribution portion.
Here, the structure of a hopper will be described in detail. A background-art hopper is a funnel-like structure in which the diameter of a top end opening portion is larger than the diameter of a bottom end opening portion as shown in Patent Literature 1. In other words, the hopper is formed out of a hollow inverted conical part and a cylindrical part extending downward from the bottom end opening portion of the inverted conical part. The hopper must have a function of allowing coarse particles classified by the classifier to fall down toward the milling table through the bottom end opening portion as described above. To this end, it is necessary to set the inclination angle of the inverted conical part at an angle which is sharp enough to prevent the coarse particles from being deposited in the hopper. That is, if the inclination angle of the inverted conical part is gentle, the coarse particles returned to the hopper by the classifier will be deposited in the hopper. It is therefore necessary to form the inverted conical part into a shape with an angle sharp enough to allow the coarse particles to fall down into the hopper.
Whether the coarse particles are deposited or not depends on the repose angle (which is an angle of a slope with which powder piled up can be kept stable without collapsing spontaneously) of the coarse particles. For example, it has been known that the repose angle of pulverized coal is about 30 degrees to 40 degrees. Therefore, the inclination angle (which is an angle from a horizontal plane) of the inverted conical part of the hopper is set at about 50 degrees in the vertical-type mill for milling coal, as shown in Patent Literature 1. Incidentally, also in the invention, the inclination angle θ of the hopper is the same as in the background art to be about 50 degrees (see FIG. 2).